The present invention relates to a roller for a roller hearth furnace in which the material is to be heated to high temperatures.
It has been known to manufacture the rollers of a ceramic material. Such rollers possess a good temperature resistance. However, for many applications they have the considerable disadvantage that ceramic materials possess only a very limited resistance to sudden changes of temperature. This assumes significance particularly in the cases when the temperature must be rapidly changed in the process of heating. Such an insufficient resistance to sudden changes of temperature is an especial disadvantage in the case when the material has to be rapidly heated, i.e., when higher heat flux densities are necessary. Then a great temperature differential exists, particularly in an inlet of the furnace, between a high temperature of the furnace and a low temperature of the material to be heated. Therefore, the rotating rollers are subjected alternately to higher and lower temperatures. Under such conditions, the ceramic rollers have an insufficient service life.
Moreover, the ceramic rollers also have the disadvantage that the rollers must have a relatively great diameter when the weight resting on them or the width of the furnace is great. This is connected with a relatively great dead weight of the rollers and a relatively low fatigue strength under repeated bending stresses thereof. As a result of a large roller diameter, heating of the workpiece from below is strongly screened or prevented entirely when the diameter of the rollers has to be substantially as large as the distance between the roller axes.
It has been further known to manufacture the rollers of heat-resistant steel or cobalt alloys. Such rollers can be utilized at temperatures up to approximately 1150.degree. C. However, for higher temperatures the strength properties of the metallic alloys are not adequate. It has been further known to cool the metallic rollers by means of a medium flowing therethrough. In this case, supporting parts of the roller which are cold may be screened by surrounding insulating means. However, this construction has a disadvantage in that the cooled heat-adsorbing and heat-conducting surfaces are very large when the diameter of the roller has to be large. The large roller diameters are necessary when the characteristics of the material to be heated do not permit it to rest on small radii of curvature. The large heat-absorbing surfaces require the installation of large heat conductors and increase the heat consumption.